In recent years, an optical communication network capable of executing data communication with a large capacity at high speeds has been expanded. It is expected that, from now on, this optical communication network will be installed in consumer appliances. In particular, for applications to transmit data among substrates in an apparatus, there have been strong demands for an optical data transmission cable (optical cable) that can be used without any change from electric cables that have been currently used. From the viewpoint of flexibility, a film optical waveguide is desirably used as this optical cable.
The optical waveguide is formed by a core having a high refractive index and a clad having a low refractive index that is placed on the periphery of the core to be made in contact therewith, and designed to transmit an optical signal that has been made incident on the core, while repeating total reflection on the border between the core and the clad. Here, the film optical waveguide has sufficient flexibility since its core and clad are made from flexible polymer materials.
When such a flexible film optical waveguide is used as an optical cable, it needs to be positioned with a photoelectric conversion element (light-receiving/emitting element) so as to be optically coupled therewith. The light-receiving/emitting element refers to an element that converts an electric signal to an optical signal so as to be transmitted, and receives an optical signal to convert it to an electric signal, and a light-emitting element is used on the light input side, while a light-receiving element is used on the light output side. This positioning process calls for precision since it gives influences to the optical coupling efficiency.
FIG. 16 shows a structural example of an optical cable module in which a film optical waveguide and a light-receiving/emitting element are optically coupled with each other.
An optical cable module 100, shown in FIG. 16, is configured by an optical waveguide 101, a light-receiving/emitting element 102 and a supporting substrate 103 that are placed on an end portion on the light-incident side or the light-releasing side. The optical waveguide 101 is secured onto the supporting substrate 103 near its end portion by bonding or the like so that the relative positional relationship between an end portion of the optical waveguide 101 and the light-receiving/emitting element 102 is in a secured state.
The supporting substrate 103 has a step difference in which the mounting face of the light-receiving/emitting element 102 and the secured face (bonding face) of the optical waveguide 101 form mutually different faces. Here, an end face of the optical waveguide 101 is not perpendicular to the optical axis (center axis in a longitudinal direction of the core), and is cut off diagonally to form an optical path conversion mirror. With this arrangement, a signal light ray, transmitted through the core of the optical waveguide 101, is reflected by the optical path conversion mirror, and changed in its traveling direction to be released toward the light-receiving/emitting element 102.
Patent Documents 1 and 2 have disclosed a structure in which the gap between a light-emitting element and an optical waveguide is filled with a resin having a high refractive index so that the optical waveguide is bonded and secured by this resin. In this structure, the resin layer suppresses an undesired interface reflection so that the optical coupling efficiency can be improved.
Patent Document 1: JP-A No. 2000-214351 (Date of Publication: Aug. 4, 2000).
Patent Document 2: JP-A No. 2000-9968 (Date of Publication: Jan. 14, 2000).
Patent Document 3: JP-A No. 2004-233687 (Date of Publication; Aug. 19, 2004).